The average amount saved into cash Isas for the last tax year (2012/13) was around £3,500, well short of the maximum allowance savers could put away of £5,640.
On average, savers using cash Isas squirrelled away around £290 a month in 2012/2013, said HMRC.
With the deadline for people to use this year's allowance (£5,760) fast approaching, Halifax is urging customers to make full use of it - especially if they have cash in their current accounts earning little interest.
Richard Fearon, head of savings at Halifax, said: "It's a great discipline to look at your finances every year to make sure they're helping you achieve your goals.
"If you can afford it, maximising the amount in your Isa every year is one of the best moves you can make. Likewise if you have money in other accounts, think about transferring the cash into your Isa before the tax year end."
The deadline for you to use this year's allowance is 5 April.
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Worryingly, Halifax revealed 66% of people still don't know what a junior Isa (JISA) is, despite their being introduced in November 2011.
Based on the adult account, a Jisa allows parents, grandparents and family friends to help put away cash tax-free for a child.
The allowance for Jisas for the current year is £3,270 but according to the bank only 10% of them were fully used in 2012/13.
Halifax estimates £5.1 billion is wasted in unused junior allowances.
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